You would be a great uncle, so they would be your great nephew
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
YesThe son and his sister = 1 son, 1 sisterThe sons and their sister = many sons, 1 sisterThe son and his sisters = 1 son, many sistersThe sons and their sisters = many sons, many sistersThough in reality this is a strange way to say it. More normal would be "The sons and daughters".
cousin
Your son's children are your grandchildren.
cousin
My sister's son is my nephew.
Your grandfather's son's son could be:you (if you are a boy)your brotheryour first cousin
Your father's sister is your aunt and her son is your first cousin.
First cousin, once removed.
Well, it depends how close you are to your husbands family. If you really want to know they are cousins-in-law.
They share a common Grandparent. That makes them first cousins.